A device of the kind described above is known through U.S. Pat. No. 3,987,689, where the two shafts are the output shafts of a vehicle differential mechanism. The device may accordingly in this case be called a differential brake. There may, however, be other instances when it is desired to counteract a certain rotational speed differential between two shafts, a typical example being the two shafts to the driven front and the rear axle, respectively, of a four-wheel-drive vehicle.
Whereas the above-mentioned patent publication shows a "speed-sensitive differential mechanism" and the present invention is applied to the more general case with two axially aligned shafts, the same general considerations apply.
Although U.S. Pat. No. 3,987,689 represents the closest known prior art, there are other publications suggesting neighbouring solutions to the problem of decreasing undesired rotational speed differential between two shafts, such as EP-A-0 350 044, U.S. Pat. No. 4,012,968, U.S. Pat. No. 3,488,980 and U.S. Pat. No. 5,087,228.
Returning to U.S. Pat. No. 3,987,689, the device shown therein has radial pistons driven by a circumferential cam on an actuator or ring-shaped piston. The hydraulic pressure created by the radial pistons is conveyed to the actuator for engaging a clutch and is discharged through a restricted bore (in the actuator).
This is a relatively simple design with advantages. However, its operating characteristics are built into the construction and may not easily be changed, especially not during use.
There is presently an increasing demand for having full control over different parameters or characteristics in a device of this kind. For example, an anti-lock system for vehicle brakes cannot work properly together with an uncontrolled differential brake. Under other working conditions it may be desirable to modify the torque transmission between the two shafts in question.
In other words it may be desirable to limit the torque transmitted between the two shafts for longer or shorter times to a lower level than what is motivated by the rotational speed differential between the shafts.